This invention relates to thermal ink jet printing devices and more particularly to thermal ink jet printheads having an extended operating temperature range over which the volume of ejected ink droplets may be controlled.
In the thermal ink jet printing process, a short duration voltage pulse is applied to the heating elements of the printhead, which raises the surface of the heating elements very rapidly. The ink in the neighborhood of the heating element is superheated and a vapor bubble is nucleated at the heating element surface. The bubble begins to expand under the influence of the high initial vapor pressure and continues to expand due to inertial effects, ejecting an ink droplet from the printhead nozzles. The pressure inside the bubble immediately begins to decrease because of the evaporation at the ink vapor interface with the heating element surface. The evaporation process extracts heat from the heated ink and the ink temperature slowly decreases. To some extent, the growth of the bubble and, therefore, the associated volume of the ejected ink droplet depend on the amount of energy available in the ink near the ink vapor interface. Only a small fraction of the input energy of the voltage pulse is utilized in nucleating the bubble and ejecting the ink droplet, the rest of the input energy goes into the printhead and its heat sink. As a result, the printhead temperature increases as the printing process continues. The higher printhead temperature causes an increase in the volume of the ejected ink droplet. Since the droplet volume is one of the variables that determines printed image quality, the quality of the printed image can change as the printhead temperature changes. Accordingly, one of the approaches to control the droplet volume is to modify the input energy to the heating elements, as the printhead temperature changes.
U.S. Pat. No. 4,490,728 discloses one practice currently in use, wherein a two part electrical pulse is applied to the heating elements of a thermal ink jet printer. The pulses comprise a single precursor pulse insufficient to vaporize the ink, followed by a nucleation pulse the produce the bubble and eject an ink droplet. A certain time delay is incorporated between the two pulses. The purpose of the precursor pulse or pre-pulse is to preheat the ink near the heating elements to provide additional energy to the bubble when it nucleates during the main pulse.
U.S. Pat. No. 5,107,276 discloses a thermal ink jet printhead that is maintained at a substantially constant, but higher than ambient, operating temperature during printing. To prevent printhead temperature fluctuations during printing, the heating elements not being used to eject ink droplets are selectively energized with electrical pulses having insufficient magnitude to vaporize ink.
U.S. Pat. No. 5,036,337 discloses a method and apparatus for controlling the volume of ink droplets ejected from thermal ink jet printheads. The electrical signals applied to the heating elements for generating droplet ejecting bubbles thereon are composed of packets of electrical pulses. The number of pulses per packet and the width and spacing therebetween are controlled in order to maintain the desired volume of the ejected ink droplets.
When a single pre-pulse is used, the duration of the pre-pulse determines the maximum temperature reached by the ink during the pre-pulse time. If this value is too high, that is, the pre-pulse is too long, nucleation is prematurely initiated which interferes with the main nucleation pulse causing droplet ejection failure. If the appropriate pre-pulse time is used, so that interference with the main pulse does not occur, the pre-pulse width or time is decreased as the printhead temperature is increased, eventually resulting in no pre-pulse before the main pulse. Thus, a single pre-pulse offers a measure of droplet volume control, but only over a relatively small temperature range of about 15xc2x0 C. Though some droplet volume control is available by prior art techniques, it is important to be able to provide droplet volume control over an extended temperature range.
It is an object of the present invention to provide an ink jet printhead having an extended temperature range over which the volume of the ejected ink droplets may be controlled.
In one aspect of the invention, there is provided a method for extending the temperature range over which the volume of ejected ink droplets from nozzles of a thermal ink jet printhead may be controlled, the printhead having a selectively addressable heating element for each nozzle to produce momentary ink vapor bubbles that eject an ink droplet when the heating elements are addressed with an ink-nucleating electrical pulse in response to data signals received by the printhead, the method comprising the steps of: sensing the temperature of the printhead; applying a plurality of non-nucleating electrical pre-pulses to the selected heating elements in response to data signals received; applying a nucleating pulse to each of the selected heating elements subsequent to the plurality of non-nucleating pre-pulses to eject ink droplets from the printhead nozzles; providing clocking signals having predetermined units per time period; and controlling the number of non-nucleating pre-pulses, the pre-pulse width, and time width between pre-pulses based upon a look-up table established to take into account the printhead temperature, the pre-pulse width and time width being variable multiple whole units of clock signals to maintain the desired droplet volume.
In one embodiment of the invention, a thermal ink jet printhead for ejecting ink droplets from nozzles therein to a recording medium in response to image data signals has a means for extending the temperature range over which the volume of the ejected droplets may be controlled, comprising: a plurality of selectively addressable heating elements, one for each nozzle, the heating elements each producing a momentary ink vapor bubble when addressed with an ink-nucleating electrical pulse representative of an image data signal; a temperature sensor; a power supply; and a control circuit for applying a plurality of non-nucleating electrical pre-pulses to selected heating elements in response to data signals received and a nucleating electrical pulse to the selected heating elements subsequent to the pre-pulses to eject ink droplets from the printhead nozzles, the control circuit including a clocking device, a controller with a look-up table, and drivers for applying electrical pulses to the heating elements, each pre-pulse width and time width between pre-pulses being predetermined whole numbers of clocking units generated by the clocking device, the look-up table containing pre-pulse widths and time widths between pre-pulses based upon the printhead temperature, the controller for selecting the pre-pulse widths and time widths between pre-pulses from the look-up table in order to maintain the desired droplet volume during a printing operation by the printhead when the printhead temperature changes.